1. Field of the Invention
The present invention relates to a technique for stabilizing measurement of spectropolarization characteristics of an object to be measured by use of a channeled spectrum.
2. Description of the Related Art
Light has properties of a “transverse wave”. Based upon the premise of three mutually orthogonal axes (x, y, z), when a propagation direction of light is assumed to be the z-axis direction, a vibration direction of the light is a direction along the x-y plane. The vibration direction of the light within the x-y plane has a bias. This bias of light is referred to as “polarization”. A biased state of light is referred to as a “state of polarization (SOP)” in this specification. Typically, the SOP varies depending upon wavelengths (colors) of light.
When light in some state of polarization is incident on an object to be measured to acquire outgoing light such as transparent light or reflected light and the object to be measured has optical anisotropy, a change in SOP is observed between the incident light and the outgoing light. Acquiring information on anisotropy of the object to be measured from the change in SOP is referred to as “polarimetry”. It is to be noted that causes of such anisotropy may include anisotropy of a molecular structure, presence of stress (pressure), and presence of a local field and a magnetic field.
A measurement in which a change in SOP between the incident light and the outgoing light is obtained with respect to each wavelength and information on anisotropy of an object to be measured is then acquired is especially referred to as “spectroscopic polarimetry”. This spectroscopic polarimetry has an advantage of acquiring a great amount of information as compared to the case of measurement by use of a single wavelength (single color). In the spectroscopic polarimetry, a device for measuring the change in SOP between the incident light and the outgoing light, namely a spectroscopic polarimeter, serves as a key device.
As fields of application of the spectroscopic polarimetry, there are known the field of spectroscopic ellipsometry, the medical field, and the like. In the field of spectroscopic ellipsometry, for example, since thickness as well as a complex refractive index of a thin film can be measured in a nondestructive and non-contact manner, spectroscopic polarimetry has been applied to optical electronic devices, examination/study of semiconductors, and the like. In the medical field, attempts have been made for early detection of glaucoma and a cancer cell since several kinds of cells have polarization characteristics.
As conventional typical spectroscopic polarimetry, rotating-retarder polarimetry and polarization-modulation polarimetry are known.
In these methods, a mechanical or electric polarization control element is used to modulate light to be measured, and from a change in spectral with the modulation, a Stokes parameter or the like is obtained.
However, the following problems and some other problems with the above methods have been pointed out: [1] a mechanical or electric drive unit is required; and [2] it is necessary to repeatedly change a plurality of spectrums while changing conditions of the spectroscopic polarimetry.
In order to solve these problems, channeled spectropolarimetry was previously contrived (cf. “Measurement of spectral distribution of polarized light based on frequency region interference method”, written by Takayuki Katoh, Kazuhiko Oka, Tetsu Tanaka, and Yoshihiro Ohtsuka, preliminary manuscript collection for 34th Academic Lecture Meeting of Hokkaido Branch of Japan Society of Applied Physics, (Hokkaido Branch of Japan Society of Applied Physics, Sapporo, 1998) p. 41).
Further, spectroscopic ellipsometry utilizing the channeled spectropolarimetry has also been reported (cf. “Spectroscopic ellipsometry using channeled spectrum”, written by Kazuhiko Oka and Takayuki Katoh, collected papers of lectures in 26th Study Session on Light Wave Sensing Technology (Light wave Sensing Technology Study Session held by Japan Society of Applied Physics, Dec. 19-20, 2000) pp. 107-114).
A configuration view of an experiment system for explaining the channeled spectroscopic polarimetry is shown in FIG. 26. As apparent from this figure, white light emitted from a xenon lamp 1 is transmitted through a polarizer 2 and a Babinet-Soleil compensator 3, to obtain a light wave having an SOP depending upon a frequency ν. Spectral distributions S0 (ν), S1 (ν), S2 (ν) and S3 (ν) of the Stokes parameters of the light wave are obtained by a measurement system 4 surrounded by a dashed line in the figure.
Light under measurement is first transmitted through two retarders R1 and R2 having different thicknesses (d1, d2) and an analyzer A, and then incident on a spectrometer 5. Here, a slow axis of the retarder R2 is inclined at an angle of 45° with respect to a slow axis of the retarder R1, while a transmission axis of the analyzer A is arranged in parallel to the slow axis of the retarder R1.
In each of the two retarders R1 and R2, a phase difference created between the orthogonal polarization components depends upon a frequency. Hence, as shown in FIG. 27, a channeled spectrum including three carrier components is obtained from the spectrometer 5 which functions as an optical spectrum analyzer. An amplitude and a phase of each of the carrier components are modulated by the spectrum distribution of the Stokes Parameters of the light under measurement. It is therefore possible to obtain each of the Stokes Parameters by execution of a signal processing with a computer 6 by use of Fourier transformation.
One example of results of an experiment is shown in FIG. 28. This is a result obtained in the case of inclining the Babinet-Soleil compensator 3 at an angle of 30° with respect to the slow axis of the retarder R1. Three solid lines respectively show spectral distributions S1(ν)/S0(ν), S2(ν)/S0(ν), S3(ν)/S0(ν) of the standardized Stokes parameters. It is thereby understood that the SOP depends upon the frequency.
As thus described, according to the channeled spectroscopic polarimetry, it is possible to obtain each spectrally-resolved Stokes Parameter by a frequency analysis (or wavenumber analysis) of characteristics of a spectral intensity. It is reasonably necessary to obtain respective retardations of the two retarders R1 and R2 prior to the frequency analysis. Here, the retardation means a phase difference created between a fast axis component and a slow axis component.
According to the foregoing channeled spectroscopic polarimetry, advantages can be obtained including that: [1] a mechanically movable element such as a rotating retarder is unnecessary; [2] an active element such as an electro-optic modulator is unnecessary; [3] four Stokes Parameters are obtained at once with one spectrum so that a so-called snap shot measurement can be performed; and [4] the configuration is simple, and thus suitable for size reduction.